Electric blasting cap



Nov. 30, 1954 T. J. MULQUEENY ELECTRIC BLASTING CAP Filed Sept. 27, 1951 JN VEN TOR. Thomas d. Mul

Queeny AGENT 2,695,563 Patented Nov. 30, 1954 tice ELECTRIC BLAS'IING CAP Thomas J. Mulqueeny, Godfrey, Ill., assigner to Olin Mathieson Chemical Corporation, a corporation of Virginia Application September 27, 1951, Serial No. 24,545

Claims. (Cl. MIZ-28) This invention relates to electric blasting caps and more particularly to an improved moistureproof closure for electric blasting caps.

Electric blasting caps usually comprise a tubular metallic or plastic shell containing suitable explosive charges, an electrical ignition system and a closure positioned in the open end or mouth of the shell. The closure must not only completely seal the blasting cap against the entrance of moisture but must also act as a support for the ignition assembly. The ignition assembly consists of a pair of lead wires passing through the closure and providing terminals which are bridged by means of a high resistance wire. It was formerly the custom to cast a sulfur plug around the lead wires, bridge the terminals extending from one side of the plug with a resistance wire and to properly position this assembly in the case. A waterproofing material such as pitch was then poured over the sulfur plug and a second sulfur plug cast over the layer of moistureprooflng material. This type of closure is not entirely moistureproof under all conditions particularly after it has been subjected to severe temperature changes. As a result, various other types of closures have been suggested. Substitution of a rubber plug for the sulfur and the pitch plugs, for instance, was suggested but such a plug does not rigidly support the ignition assembly. It has also been suggested that the electric blasting cap be closed by means of a'molded pre-grooved dielectric plug surrounded by an elastic material which will tenaciously adhere to both the dielectric plug and inner wall of the case. The case is crimped into the grooves formed in the dielectric plug to provide a moistureproof seal. Caps provided with this type of closure present the problems introduced by the utilization of tacky elastic materials and the requirement that the annular crimp mesh with the pre-formed grooves in the dielectric plug.

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It is therefore an object of this invention to provide an electric blasting cap comprising a novel moisture impervious closure plug. It is a further object of this invention to provide a moisture impervious closure for electricl blasting caps which is sufficiently rigid to support the electrical firing assembly and can be installed within the cap with facility. Another object of this invention is to provide an improved moisture impervious electric blasting cap which is not susceptible to ignition by static electricity. Still another object of this invention is to provide a method of making a moisture impervious closure for electric blasting caps which is effective and yet can be accomplished with facility. A still further object is to provide a blasting cap having an improved closure.

In accordance with this invention, the foregoing objects are attained by sealing the open end or mouth of an electric blasting cap by means of a resilient gasket compressed between the inner wall of the shell and a rigid flanged plug member which supports the ignition assembly. The rigid plug member is formed about the leg wires and comprises a flange portion having external dimensions such that it forms a close fit with the inner wall of the shell and a stem portion of lesser diameter. The rigid flanged plug member can be conveniently assembled with the resilient gasket prior to insertion into the explosively charged shell. After the assembly has been positioned within the shell, the closure is completed by the relatively simple operation of forcing the Such closures similarly y lack the rigidity required to support the ignition assem- M shell wall into the gasket material by crimping, thus radially compressing the resilient gasket between the rigid plug and the shell wall and effecting moistureproof junctures therebetween. In order to achieve maximum moistureproofing, preferably more than one annular crimp, or a corrugated crimp or similar indentations are formed in the wall of the shell.

By constructing the plug with a flange portion a closure is obtained having the advantages of resilient plugs as well as the advantages of the heretofore utilized rigid plugs such as sulfur and the like. The flange portion serves to center and align the ignition assembly in the shell and gives rigidity to the plug facilitating assembly. In addition, when a powdered or granulated igniting charge is utilized, the flange portion of the plug compresses the igniting charge and retains it properly positioned in the shell without migration or exudation of any of the powdered explosive to a position between the gasket and plug or between the shell wall and the closure thus precluding the possibility of an explosion during crimping due to compression of explosive materials by the crimp. Likewise, any possible increase in the impact sensitivity of the cap due to the presence of explosives between the various components of the closure is avoided. Such advantages are not available with prior closures in which a gasket type closure is utilized or with prior resilient closures.

The various objects of the invention will be clear from the description which follows and from the drawings, in which Figure l is a longitudinal sectional drawing of an electric blasting cap constructed in accordance with one of shell 1.

embodiment of this invention;

Figure 2 is an enlarged perspective view of a flanged rigid dielectric plug which is one component of the novel eledctric blasting cap closure provided by this invention; an

Figure 3 is an enlarged perspective view of one type of resilient gasket which is another component part of the closure.

Referring now to Figure 1 of the drawing for a more detailed description of one embodiment of this invention, a metallic shell or case 1 containing a base charge of explosive 2, an initiating charge 3 and an igniting charge 4 is provided in accordance with usual practice. A molded plug 5 with a flange portion 6 having external dimensions such that a free fit will be produced with the inner wall of the shell 1 and a stern portion 7 of lesser diameter than said flange portion is positioned above said igniting charge 4. Lead wires 8 extend through the plug 5 and project into the igniting charge 4 where they are bridged by means of a resistance wire 9.

A resilient gasket 10 is provided about the stem portion 7 and above the flange portion 6 of the plug 5. The wall of shell 1 is provided with annular crimps 11 and 12 which compress gasket 10 between the wall of shell ll and the rigid plug 5. It will be noted that the flange portion 6 supports the gasket 10 and is in turn itself supported by the explosive charges 2, 3 and 4 thus insuring a compact package and contact of the resistance wire 9 with the igniting charge 4. The flange base 6 of the dielectric plug 5 also facilitates insertion of the closure into the mouth of the shell by maintaining the complete closure assembly including the ignition assembly embedded therein in alignment with the wall Best results are obtained if an annular crimp l1 is formed near the mouth of the shell 1 and one or more additional annular crimps such as 12 are formed thereafter between the first crimp and the flanged base 6 of the plug 5, as shown. If this crimping process is followed, the annular crimp 11 in combination with the flanged base portion 6 of the plug 5 anchors the gasket 10 against longitudinal movement relative to the plug 5 thus insuring maximum radial compression of the gasket 10 by crimp 12 and a moistureproof juncture between the gasket 10 and the shell l.

It is to be noted that a closure for electric blasting caps constructed in accordance with this invention is particularly well adapted for elimination of the hazards involved with caps which are susceptible to ignition by static electricity. For instance, powdered or granular materials such as graphite, carbon black, aluminum, silver, iron or similar high resistance conductors of electricity can be distributed throughout the molded plug '5 for only 'distributed throughout the flange portion 6 'of plug 5. Static electricity will be caused to pass from fthe bared lead wires embedded in the 'plug 5 to the shell wallthrough the conducting material in the ange `6 and thus `prevent premature ring of the explosives due to formation of a spark gap Within the locus of the igniting charge 4. Alternately, due to the `reduction in distance between the bared lead wires v8 and the wall of shell a spark gap of less resistance than that 'formed within the locus of the igniting charge 4 between the ignition circuit and wall of shell 1 .can be readily provided. ln addition, if desired, the uninsulated portions of the lead wires can be kinked or deformed prior to embedding inplug S to provide a spark gap of even less resistance than that provided by one or more of the crimps 11 or 12 shown .in lFigure 1.

The gasket 10 is `preferably tubular and of such dimensions as to be .in frictional engagement with the inner wall of the shell and the periphery of stem `7. It may vbe formed from any resilient, soft, non-porous material such as natural rubber, one of the synthetic rubbers, for example, neoprene or any soft resilient resinous material. Graphite or carbon black may be embedded in .the `gasket to reduce the electric blasting caps susceptibility to static electricity.

The plug 5 is preferably molded from an `organic plastic material compatible with the yinsulation on `the lead wires to insure a moistureproof juncture therebetween. For example, any one or combination of one or more of the following materials which yis `compatible with 4the lead wire insulation might be utilized to advantage for forming the plug 5 about the lead wires: polystyrene, ethyl cellulose, polyvinyl chloride, urea vformaldehyde phenol-formaldehyde resins, or the like.

It should be apparent from the foregoing description that the invention accomplishes its objects and provides a novel moisture impervious .closure for electric blasting caps which can 4be easily assembled and which affords sufficient support for the ignition assembly .to .insure proper functioning of the cap.

While embodiments of this invention .have been described in detail, it is to be understood that various modications will occur to those skilled in the art and that this invention is not limited to the specic embodiments or various modifications set rforth hereinbefore, except as indicated in the appended claims.

Having thus described the invention, what is claimed and desired to secure by Letters Patent is:

l. In .an electric blasting cap having -a .metallic shell, explosive charges therein, conductors Ientering said shell and bridged with a resistance element, a closure positioned in the mouth of said shell comprising a rigid plug having dispersed therein particles of an electrical conductor, said plug being molded about said lead wires and -having a flange portion forming a free fit with the inner wall of said shell and underlying a resilient gasket having a portion thereof compressed between indentations in the Ishell wall and the substantially `smooth vsurface of a stem 4portion of said plug, `said stem portion being exposed at the mouth of said shell.

'2. 4AIn an Velectric blasting cap having .a metallic shell, explosive charges therein, conductors entering said shell and bridged with 'a resistance element, a lclosure positioned in the mouth of said shell comprising a rigid plug molded about vsaid lead wires and having a `iiange portion forming a free t with the .inner wall of said shell and underlying a resilient gasket having dispersed therein particles o'f an electrical conductor and having a portion thereof compressed between indentations .in thc shell wall and thesubstantially smooth surface of a stem portion of said plug, said stem portion being exposed atfthe mouth of said shell.

VIn an electric blasting cap having a metallic shell, explosive charges therein, conductors entering said shell and bridged with a resistance wire, a moisture impervious closure positioned in the mouth of said shell which comprises a rigid plug molded about said lead wires and having a ange portion forming a free Iit with the inner wall of said shell and underlying a resilient gasket radially compressed between a substantially smooth stem portion and indentations in the shell wall, the length of the path between a .portion of at least `one of said conductors embedded in said plug and said shell wall being substantially lless than the length of any path vbetween the resistance element and shell wall, thus providing a spark gap between the shell wall and at least one -of `the conductors at a point outside the locus of the ignition charge having .a resistance substantially :less than the resistance across the spark gap between the ignition circuit and the shell wall at the locus of the ignition composition, whereby static electricity will jump across the gap having the lesser resistance and pass harmlessly to the shell wall.

4. -In the -manufacturefof electric blasting caps wherein a shell having an open end is charged with explosives andthe open-'end is closed-by a plug through which conductors extend, the 'improvement lwhich comprises molding the plug about a pair of vconductors in a shape providing a flange portion of diameter lsuch as to form a free Vfit within the 'shell and an integral stern portion of lesser diameter, providing a tubular gasket of relatively soft resilient materiaLsaid gasket having a central opening of size such as to be received over `said stem and having an exterior dimension such as to be freely received within said shell, yinserting said plug in the gasket and inserting the resulting assembly flange 'rst in the shell, and then, after said parts are centrally positioned by means Vo'f said lange, crimping `the wall of said shell into sai'd gasket to'compress the gasket 'into sealing relationship lbetween the interior of the shell and the exterior of the stern to form a moisture 4impervious seal therebetween, and to .secure the plug within said shell.

5. In an electric blasting cap having a shell, an explosive charge in the shell, and conductors rextending through `a closure for the shell, the improvement which comprises `a Aplug of vrelatively rigid material having a substantially smooth surfaced stem portion of substantially lesser-outside dimension `than the inside dimension of the shell and having a ange portion extending radially outward of the stem Aportion at one 'end of the latter, said ange portion having a lfree lit =within 'the shell, said plug having Ybeen molded about said conductors and said plug being disposed within Vsaid shell with the ange portion thereof at a substantial distance 'from the mouth of said shell and `the stern portion projecting outwardly therefrom, and a soft resilient rubber-like tubular gasket above saidfangeportion and 'having a central opening of size such as to'have a free fit over said stem and vhaving an exterior dimension such as to tit freely -within said shell, and indentations in the shell wall compressing said gasket "between said stem portion and the inner wall of said shell, thus forming a moisture impervious Vseal therebetween and securing the plug within said shell.

References Cited in the iile of this patent UNITED 4STATES PATENTS Numb er Name .Date

'1,590,364 Grant llune '29, 1926 1,695,825 Schurmann nDec. 18, 1'92'8 2,103,432 Nash Dec. '28, 1937 2,356,337 Miller Aug. 22, 1944 2,400,103 'Cobb `May 14, 1946 2,428,884 Lefren Oct. '14, 1947 2,477,458 Johnson yJuly 26, 1949 2,478,415 Miller Aug. 9, 1949 FOREIGN .PATENTS Number "Country Date V215,222 'Great Britain. of 1905 555,969 Great Britain Sept. .15, 1943 

